For communications and jamming systems which use digital processors to packetize data and to process or reconstruct the information to be transmitted at a precise instant of time, not only must stored packets of data be transmitted at exactly the right start time, it is oftentimes necessary to change the characteristics of the transmitter to support a particular application.
For instance, in certain types of communications systems, the frequency of the transmitter is frequency-hopped instant by instant so as to provide for covert operations as well as to provide for spread spectrum operation. In other types of communications systems such as code division multiple access, CDMA, systems, these systems are operated so that signals are transmitted simultaneously on the same frequency and are coded such that they can be disambiguated or retrieved regardless of the fact that they are transmitted at the same time and on the same frequency. Oftentimes, the CDMA systems require that the frequency at which the data is transmitted be momentarily shifted when, for instance, the CDMA system cannot effectively operate to separate out the signals on the same frequency. Under these circumstances, it is sometimes required to change the frequency of the transmission on the fly. This requires precise control of the frequency of the transmitter which must be done on a realtime basis, meaning to nanosecond accuracies.
Frequency hopping is also used for radars in which the output frequency must be rapidly changed in synchrony with the pulses. Also, pulse repetition rates, and modulation type of the particular transmissions are altered on the fly for various purposes. Thus, there is a whole class of frequency agile radars which are used for a wide variety of purposes, as well as radars whose transmit characteristics in terms of modulation type and pulse repetition frequencies need to be varied at will. When the transmission of these radars involves the transmission of packetized data, it is important to be able to synchronize the frequency shifts or changes in modulation type and/or amplitude to occur in synchronization with a precisely timed transmission of the packet.
There is also an application for the use of precision timed transmission of packets in the field of jamming in which multi-purpose jammers must be able to multiplex between various frequencies, modulation types, amplitudes and other factors so as to be able to be effective against a large variety of targets.
Key to any of the above communications, radar and jamming systems is the ability to synchronize the transmission of packetized data with how the packet itself is to be transmitted.
When it is possible to transmit packetized data a precise moment of time controlled to the nanosecond, it is likewise important to be able to reconfigure the transmit chain so as to be able to transmit the packetized data with the appropriate frequency, amplitude or modulation type.
In a co-pending U.S. Patent application entitled, Method for Realtime Digital Processing of Communication Signals by Robert Boland, assigned to the Assignee hereof and incorporated herein by reference, a system is described for using standard off-the-shelf components for the creation of the digitally processed signals used for so-called software radios.
In this patent application, a system is described to eliminate the need for high-cost, difficult to develop, specific digital hardware and realtime synchronizer software for the digital processing by substituting general purpose digital processors which operate in non-realtime, usually having clock accuracies in the millisecond range.
In order to be able to precisely time the output of packetized data which is stored in a buffer, these systems are provided with a precision timed output gate that is controlled by a precision time reference. The precision timing is provided by the gate and reference which constitute a realtime interface that is coupled to a buffer to buffer the output from the standard digital processor.
The digital processor performs two functions. First, it processes and packetizes the data to be transmitted and then it provides the packet with a time header that specifies the exact time at which the packet is to be transmitted. The use of the realtime interface permits the use of standard off-the-shelf processors without having to pay particular attention to precision synchronization and timing, with the interface providing the precision timing necessary to transmit the packetized data at a particular instant in time.
Prior to the invention described in the co-pending application, in order to provide for the realtime accuracy, the entire radio and all of its components were required to have precise timing standards and specialized hardware or software components which assured that for each of the components in the entire communication chain all signals were synced together with the required accuracy.
This required the costly development of extremely specialized digital hardware and realtime synchronizing software to be able to synchronize all parts of the communication chain to establish the exact point in time that the particular packet is transmitted.
Part of the communication chain also involves the transmit chain which includes a transmitter and antenna, with the transmitter also being able to alter the amplitude and frequency of its transmissions as well as having a modulator whose type can be changed on the fly.
As in the case with the exact timing of the transmission of the packets, the changing of the transmit chain by changing the frequency, amplitude and modulation type of the transmitter also had to be performed in precise synchronization with the transmission of the particular packet. As with the precise timing of the transmission of the packet, the timing of the change over of the transmission chain also had to be accomplished in realtime with accuracies commensurate with the accuracy at which the packet was to be transmitted.
Again, as with the exact timing of the transmission of the packet, the control of the transmit chain was inextricably dependent upon the development of precise time references for all components in the system and therefore required the development of specific digital hardware and the ability to control the change over in the transmit chain in realtime.
Note for the purposes of the present invention, realtime means higher accuracy than that available from standard off-the-shelf products, for instance, nanosecond accuracy.
Whether generating the packet and transmitting it at an extremely precise time, or whether reconfiguring the transmit chain to transmit the packet at the required frequency, amplitude or modulation type, the use of standard off-the-shelf components was precluded because timing accuracy was insufficient.